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Optimised High Frequency Transformer Design with Arbitrary Current Waveforms

Switching circuits, operating at high frequencies, have led to considerable reductions in the size of magnetic components and power supplies. Non-sinusoidal voltage and current waveforms and high frequency skin and proximity effects contribute to transformer losses. Traditionally, transformer design has been based on sinusoidal voltage and current waveforms operating at low frequencies. The physical and electrical properties of the transformer form the basis of a anew design methodology while taking full account of the type of current and voltage waveforms and high frequency effects. Core selection is based on the optimum throughput of energy with minimum losses. The optimum core is found directly from the transformer specifications: frequency, power output and temperature rise. The methodology is suitable for use in a computer application in conjunction with a database of core and winding materials. High frequency AC loss effects must then be taken into account. The AC losses due to non-sinusoidal current waveforms have traditionally been found by calculating he losses at harmonic frequencies when the Fourier coefficients are known. An optimised foil or layer thickness in a winding may be found by applying the Fourier analysis over a range of thickness value. New methodologies have been developed to find the optimum foil or layer thickness for any periodic waveshape, without the need for calculation of AC losses at harmonic frequencies. The first methodology requires the RMS value of the current waveform and RMS value of its derivative. The second methodology makes use of regression analysis and some harmonic summations.